expt: repair for non-integer power of negative inexact

If the inexact approximation of the power is an integer, then
the result was a real number when it should be a complex number.

pkgs/racket-pkgs/racket-test/tests/racket/number.rktl

@@ -566,6 +566,10 @@
 (test 0.0f0 expt -0.9999f0 (expt 2 5000))
 (test -0.0f0 expt -0.9999f0 (add1 (expt 2 5000)))
 
+(test +inf.0+inf.0i expt -0.1 -1000000000000001/3) ; inexact approx is non-integer
+(test -inf.0-inf.0i expt -0.1 -100000000000000000001/3) ; inexact approx is integer
+
+
 (err/rt-test (max 0 'a))
 (err/rt-test (min 0 'a))
 (err/rt-test (max 'a 0))

racket/src/racket/src/number.c

@@ -3618,6 +3618,13 @@ scheme_expt(int argc, Scheme_Object *argv[])
           return SELECT_EXPT_PRECISION(scheme_nzerof, scheme_nzerod);
       }
     }
+
+    if ((d < 0.0) && SCHEME_RATIONALP(e)) {
+      /* The inexact approximation of a rational number might be an integer.
+         Make sure we stay on the complex track: */
+      return scheme_complex_power(scheme_real_to_complex(n),
+                                  scheme_real_to_complex(e));
+    }
   }
 
   r = bin_expt(argv[0], e);